R e V i e w : n e u r o s c I e n c e the Faculty of Language: What Is It, Who Has It, and How Did It Evolve?

tion of analysis of fossil skull shape and cranial endocasts has led to little consensus about the evolution of language (7, 9). A
more tractable and, we think, powerful approach to problems of language evolution is provided by the comparative method, which uses empirical data from living species to draw detailed inferences about extinct ancestors (3, 10 –12). The comparative method was the primary tool used by Darwin (13, 14 ) to analyze evolutionary phenomena and continues to play a central role throughout modern evolutionary biology. Although scholars interested in language evolution have often ignored comparative data altogether or focused narrowly on data from nonhuman primates,
current thinking in neuroscience, molecular biology, and developmental biology indicates that many aspects of neural and developmental function are highly conserved, encouraging the extension of the comparative method to all vertebrates (and perhaps beyond. For several reasons, detailed below, we believe that the comparative method should play a more central role in future discussions of language evolution.
An overarching concern in studies of language evolution is with whether particular components of the faculty of language evolved specifically for human language and,
therefore (by extension, are unique to humans. Logically, the human uniqueness claim must be based on data indicating an absence of the trait in nonhuman animals and, to betaken seriously, requires a substantial body of relevant comparative data. More concretely,
if the language evolution researcher wishes to make the claim that a trait evolved uniquely in humans for the function of language processing, data indicating that no other animal has this particular trait are required.
Although this line of reasoning may appear obvious, it is surprisingly common fora trait to beheld up as uniquely human before any appropriate comparative data are available. A famous example is categorical perception, which when discovered seemed so finely tuned to the details of human speech as to constitute a unique human adaptation (15,
16 ). It was sometime before the same underlying perceptual discontinuities were discovered in chinchillas and macaques (17, 18),
and even birds (19), leading to the opposite conclusion that the perceptual basis for categorical perception is a primitive vertebrate characteristic that evolved for general auditory processing, as opposed to specific speech processing. Thus, a basic and logically in- eliminable role for comparative research on language evolution is this simple and essentially negative one A trait present in nonhuman animals did not evolve specifically for human language, although it maybe part of the language faculty and play an intimate role in language processing. It is possible, of course, that a trait evolved in nonhuman animals and humans independently, as analogs rather than homologs. This would preserve the possibility that the trait evolved for language in humans but evolved for some other reason in the comparative animal group. In cases where the comparative group is anon- human primate, and perhaps especially chimpanzees, the plausibility of this evolutionary scenario is weaker. In any case, comparative data are critical to this judgment.
Despite the crucial role of homology in comparative biology, homologous traits are not the only relevant source of evolutionary data.
The convergent evolution of similar characters in two independent clades, termed “analogies”
or “homoplasies,” can be equally revealing
(20). The remarkably similar (but nonhomolo- gous) structures of human and octopus eyes reveal the stringent constraints placed by the laws of optics and the contingencies of development on an organ capable of focusing a sharp image onto a sheet of receptors. Detailed analogies between the parts of the vertebrate and cephalopod eye also provide independent evidence that each component is an adaptation for image formation, shaped by natural selection.
Furthermore, the discovery that remarkably conservative genetic cascades underlie the development of such analogous structures provides important insights into the ways in which developmental mechanisms can channel evolution (21). Thus, although potentially misleading for taxonomists, analogies provide critical data about adaptation under physical and developmental constraints. Casting the comparative net more broadly, therefore, will most likely reveal larger regularities in evolution, helping to address the role of such constraints in the evolution of language.
An analogy recognized as particularly relevant to language is the acquisition of song by birds (12). In contrast to nonhuman primates,
where the production of species-typical vocalizations is largely innate (22), most songbirds learn their species-specific song by listening to conspecifics, and they develop highly aberrant song if deprived of such experience. Current investigation of birdsong reveals detailed and intriguing parallels with speech (11, 23, 24).
For instance, many songbirds pass through a critical period in development beyond which they produce defective songs that no amount of acoustic input can remedy, reminiscent of the difficulty adult humans have in fully mastering new languages. Further, and in parallel with the babbling phase of vocalizing or signing human infants (25), young birds pass through a phase of song development in which they spontaneously produce amorphous versions of adult song, termed “subsong” or babbling Although the mechanisms underlying the acquisition of birdsong and human language are clearly analogs and not homologs, their core components share a deeply conserved neural and developmental foundation Most aspects of neurophysiology and development—including regulatory and structural genes, as well as neuron types and neurotransmitters—are shared among vertebrates. That such close parallels have evolved suggests the existence of important constraints on how vertebrate brains can acquire large vocabularies of complex, learned sounds. Such constraints may essentially force natural selection to come up with the same solution repeatedly when confronted with similar problems.

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